Levels and sub-cellular distribution of physiologically important metal ions in neuronal cells cultured from chick embryo cerebral cortex

Abstract

Mg²⁺, Ca²⁺, Mn²⁺, Zn²⁺, and Cu content of neurons from chick embryo cortex cultivated in chemically defined serum free growth medium was determined by energy dispersive X-ray fluorescence and atomic absorption spectroscopy. The intracellular volume of cultured neurons was determined to be 2.73 μl/mg. Intracellular Mn²⁺, Fe²⁺, Zn²⁺, and Cu²⁺ in the cultivated neurons were 100–200 times the concentrations in the growth medium: Mg²⁺ and Ca²⁺ were 0.9 and 1.7 mM respectively, around 20 fold higher than in growth medium. Mg²⁺, Fe²⁺, Cu²⁺ and Zn²⁺ concentrations in neurons were in the range of ca. 300–600 μM, approximately 2–3 times the values previously reported in glial cells; Ca²⁺ and Mn²⁺ content of the neurons were higher by 5 and 10 fold respectively compared to glial cells. In neurons, the subcellular distribution of Fe²⁺, Cu²⁺, and Mn²⁺ follows the rank order: cytosol>microsomes>mitochondria; for Zn²⁺ the distribution differs as following: cytosol >mitochondria>microsomes. Determination of the superoxide dismutase activities in the cultivated neurons indicated that the Mn²⁺ linked activity predominates whereas, the Cu-Zn dependent enzyme is dominant in glial cells. Enrichment of the culture medium with Mn²⁺ to 2.5 μM enhanced the Mn-SOD by approximately 33% but Cu²⁺−Zn²⁺ enzyme activity was not modified. The high Mn²⁺ content, the capacity to accumulate Mn²⁺, and the predominancy of the Mn−SOD form observed in neurons is in accord with a fundamental functional role for this metal ion in this type of brain cells.